Verizon versions are sim unlocked out of the box but T-Mobile is not. You need to contact TMobile and if you meet their conditions (fully paid off, 40 days on their network etc) you can only sim unlock via TMobile unlock app on the phone.
Other than that, you need to check what LTE bands Aus networks use and if the phones have all the necessary bands to have great connection anywhere you go. Now a days most phones have all the necessary radios to cover most part of the world but just double check if there is any unique odd bands that Aus networks use.
Also, you are running Verizon and T-Mobile firmware so OS/security updates are dependent on this and only you will get it when the updates are ready for these particular versions and pushed by TMobile or Verizon.
If I was an Aussie buyer I wouldn't touch the US carrier version because they run crippled US carrier firmware and also SD 835 processor compared to exynos processor in most part of the world. But if the buyer is uninformed about all these, and you are selling at a good price you might find people easily to buy.